The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
LIDAR is increasingly used for distance measurement in a variety of applications, ranging from cartography to microscopy. The incorporation of such functionality into an ever-growing range of devices, including autonomous or semi-autonomous vehicles has led to the development of increasingly compact and low power consumption devices. At the same time, demand for LIDAR devices with increased range and improved resolution continues to grow.
LIDAR can be accomplished in a variety of ways. In “time of flight” (TOF) LIDAR short pulses of light are emitted and reflected pulses received, with the delay between emission and reception providing a measure of distance between the emitter and the reflecting object. Such TOF systems, however, have a number of disadvantages. For example, simple TOF measurements are highly susceptible to interference from other signal sources. This issue becomes more pronounced as the distance between the emitter and the reflecting object increases, as such distance necessarily decreases the strength of the reflected signal. On the other hand, inherent limitations in accurately measuring extremely short time intervals limit the spatial resolution of such TOF LIDAR systems at close range. In addition, the range of such TOF LIDARs is a function of the ability to detect the relatively faint reflected signal. The resulting range limitations are frequently addressed by using highly sensitive photodetectors. In some instances such detectors can detect single photons. Unfortunately this high degree of sensitivity also leads to increased misidentification of interfering signals as reflect TOF LIDAR pulses. Despite these disadvantages TOF LIDAR systems currently find wide application, primarily due to the ability to provide such systems in a very compact format and the ability to utilize relatively inexpensive non-coherent laser light sources.
Alternatives to TOF LIDAR have been developed. One of these, frequency modulated (FM) LIDAR, relies on a coherent laser source to generate repeated brief “chirps” of time delimited, frequency modulated optical energy. The frequency within each chirp varies linearly, and measurement of the phase and frequency of an echoing chirp relative to a reference signal provides a measure of distance and velocity of the reflecting object relative to the emitter. Other properties of the reflected chirp (for example, intensity) can be related to color, surface texture, or composition of the reflecting surface. In addition, such FM LIDARs are relatively immune to interfering light sources (which tend to produce non-modulated signals) and do not require the use of highly sensitive photodetectors.
The accuracy of this measurement depends upon a number of factors, including the linewidth limitations of the emitting laser, the range of frequencies (i.e. bandwidth) within the chirp, the linearity of the frequency change during each chirp, and the reproducibility of individual chirps. Unfortunately, improvement in one of these factors is generally at the detriment of the remaining factors. For example, while increasing the bandwidth of the chirps improves resolution, doing so makes it difficult to maintain linearity of the frequency change within the chirp. Similarly, lasers that have a narrow linewidth can be poorly suited for production of the range frequencies required to generate a chirp. In addition, FM LIDAR systems that have been developed to date are far from compact, as they rely on relatively large FMCW laser sources. In addition, such systems typically rely on a carefully modulated, low noise local oscillator (for example, a narrow linewidth solid state, gas, or fiber laser) with frequency modulation corresponding to that of the emitted chirp provided by a relatively large interferometer. This local oscillator precisely replicates an emitted chirp, and serves as the reference for the received reflected chirp. As a result FM LIDARs are relatively large, complex, and expensive, and have seen limited implementation relative to TOF LIDARs despite their performance advantages.
Quack et at (presentation at GOMACTech, St. Louis, Mo., USA, Mar. 23-26, 2015) have proposed development of a FMCW LIDAR source device that would require construction and integration of an electromechanically modulated laser source, an optical interferometer, and modulating electronics on a single silicon chip. The resulting device however, relies on an electronic feedback system that inherently generates nonlinear optical chirps. This is only partially corrected by applying a “pre-distorting” the feedback signal supplied to the laser source and utilization of an external reference frequency generator that can act as an additional source of variation (Satyan et al, Optics Express vol. 17, 2009). The resulting LIDAR source is highly complex, and it remains to be seen if such diverse features can be successfully integrated on a single silicon chip in a reliable fashion. In addition, a LIDAR incorporating such a source is still reliant on the use of a complex local oscillator to provide useful data.
Thus there is a need for a compact, robust, and efficient LIDAR system that exhibits a high degree of chirp linearity, large chirp bandwidth, and high chirp reproducibility.
The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.